Always say thank you, even if you don't want to......You will probably hear that a LOT. My poor daughter did, because we both have dark blue eyes, high cheeks, etc. It doesn't bother her, but I used to HATE it when somebody said that to me. hahaha
You can tell him thank you. Many daughters take after their mom and it is a natural thing. If you like him, you can keep the conversation going and by the end of it, you will detect if he is interested in you or not.
Well if your mom is a good looking lady for her age, say thank you! If she isn't just be polite, thank the person, and move on! 3 of my kids had this happen to them all of the time! People would always say how much the looked like their dad. We would just smile, thank them, and walk away! All my children were taken in because their bio-daddy didn't want them, I was the very lucky guy who raised them!